Sounds like you got lucky on the barrel - even better if it's got the rifling.
On any original muzzleloader, I have always removed the barrel, tied it to a tire with a load and set it off with firecracker/cannon fuse to test it (standing back mind you). This same procedure is/was outlined in the back of Dixie Gun Work catalogs many years ago.
Remember that you are dealing with a firearm that is 150 years plus old. Check the breech plug threads to make sure they are still good and look for anything that could cause a problem. Chances are, you will be just fine.
As already stated - service load with an approx 500 gr. minie ball was 60 gr of FFG. Just remember that the gun is an antique and not meant to be loaded with heavy loads and there is no need to use anything over a service load. Chances are, with a chopped barrel length, you might even end up with unburned powder with a 60 grain load - all depends on the barrel, minie ball fit, etc. Out of my 58 cal. rifled muskets - both originals and reproductions - I have shot everything from round ball, new style minie, old style minie and the wadcutter. The wadcutter is a lighter weight bullet, requires less powder and can be very accurate at 25 to 50 yards or so.
Some may "poo poo" what I'm saying but it's better with originals such as this to be on the safe side and start with milder loads and go from there. Just use common sense and you'll be fine.
If you decide you want to cast for the rifled musket - you can sometimes find some good deals on Lyman mold blocks on eBay (mold #575213) or you can get cheaper Lee molds from Tital Reloading. Use soft lead so the skirt of the slug can expand and grip the rifling. You can use any good BP lube but all I've ever used is Crisco in the base and that works just fine to keep fouling soft.
Have fun and let us know how it goes - be safe!